The present invention relates to equipment of the type used in surveying and construction and, more particularly, to a detector device for such applications which has an improved photodetector arrangement for detecting the position or level of a reference plane. The plane is typically defined by a rotating laser beam or by a stationary plane of laser light.
Laser systems have commonly been employed in surveying and construction in which a laser beam is rotated in either a horizontal or a graded plane. U.S. Pat. No. 4,062,634, issued Dec. 13, 1977, to Rando, illustrates a laser projector device which provides such a rotating reference beam. The rotating beam defines a plane, and various measurements can be made using the plane as a reference. For example, the elevation of a point remote from the laser beam projector device may be measured by means of a rod on which a laser beam detector is mounted. The bottom of the rod rests on the ground at the point where the measurement is to be made, and the operator moves the detector along the rod to a position where it intercepts the laser beam, as indicated by a display on the detector device. One such detector device is shown in U.S. Pat. No. 4,240,208, issued Jun. 30, 1987, to Pehrson.
A similar surveying system is shown in U.S. Pat. No. 4,732,471, issued Mar. 22, 1988, to Cain et al. In the Cain et al. system, a rotating beam is not used. Rather, a laser transmitter produces an alignment field by projecting laser energy in a non-planar, stationary reference cone. The Cain et al. patent discloses a small, hand-held device which includes a display and a photodetector module.
The detector device shown in the Cain et al. patent includes a pair of adjacent, triangularly shaped photodetector elements. The orientation of the photodetector elements is such that the changes in the signal outputs from the elements which occur due to relative vertical movement of the reference light are inversely related. As the reference light moves upward, the signal output from one of the elements increases while the signal output from the other of the elements decreases. The opposite signal changes occur when the position of the reference light moves downward with respect to the detector device. Naturally, the signal levels are also affected by the intensity of the light source and the distance of the photodetecor elements from the light source. By comparing the relative signal output levels from the two photodetector elements, however, it is possible to obtain an indication of the position of the reference light without regard to the absolute intensity of the light. If desired, the ratio of the output signal levels may be categorized as falling into one or more ranges corresponding to one or more vertical position ranges, as shown in U.S. Pat. No. 4,676,634, issued Jun. 30, 1987, to Petersen.
Although a detector device including a pair of triangular photodetector elements provides accurate operation under varying ambient conditions, it was found that such a device is subject to error in instances where it does not directly face the laser beam projector. Because the casing of the detector device defines an aperture behind which the photodetector elements are positioned, when the beam is not normal to the photodetector elements, a portion of one of the photodetector elements near the edge of the aperture may be placed in shadow. It will be appreciated that this shadow will reduce the output signal from the affected photodetector element, thereby also affecting the ratio of the outputs from the two photodetector elements.
This source of inaccuracy was addressed in U.S. Pat. No. 4,907,874, issued Mar. 13, 1990, to Ake. This patent discloses a detection and display device that includes first and second interdigitated photodetector elements positioned adjacent each other. The photodetector elements are each made up of a plurality of sections which are arranged in a generally vertically oriented row. The heights of the sections of one of the elements increase from the bottom of the row to the top of the row, while the heights of the sections of the other element decrease from the bottom of the row to the top of the row. This arrangement has the advantage that shading of the elements along a vertical edge of the aperture does not affect the relative areas of the elements illuminated by a beam of light.
One limitation with the detection and display device of the '874 patent is that if the beam is too narrow and too few of the sections of the elements are illuminated, a non-uniform response may occur. Further, if the width of the beam happens to be such that as the beam is shifted upward, the top of the beam is shifted onto an area of one element while the bottom of the beam is shifted off of an area of the same element, no change in relative output ratio will occur. The relative output ratio will only change when the beam is moving off of an area of one element while it is moving onto an area of the other element. As a consequence, a stair-step cell output ratio is provided, instead of the desired continuously varying response.
To deal with these problems, an enhanced detection and display device was developed, as shown in U.S. Pat. No. 4,976,538, issued Dec. 11, 1990, to Ake. The device of the '538 patent has first and second interdigitated photoconductor elements that are each made up of a plurality of sections arranged in a generally vertically oriented row. In the same manner as shown in the '874 patent, the heights of the sections of one of the elements increase from the bottom of the row to the top of the row while the heights of the sections of the other element increase from the top of the row to the bottom of the row. Unlike the sections of the device of '874 patent, however, the sections of the elements in the device of the '538 patent are inclined with respect to the row. As a consequence, a beam sweeps across the sections in a manner providing a continuously vary output ratio and across sections of both elements.
While the various detection devices discussed above provide accurate detection of laser light in a reference plane, they all require the use of specially constructed photodetector elements having specific design geometries. Such photodetector elements are expensive, and add significantly to the overall cost of the detection devices. General purpose, photodetector elements in the form of PIN photodiodes are available at a fraction of the cost. It is desired to incorporate such low cost photodetector elements into a detection device, while providing output signals having a continuously varying ratio which indicates beam position.